Optical spinel articles and methods for forming same

ABSTRACT

A single crystal spinel material is disclosed, the material having a non-stoichiometric composition and having a transparency window over a wavelength range of about 400 nm to about 800 nm. According to an embodiment, the transparency window is defined as the largest single absorptivity peak height along the wavelength range to be not greater than 0.35 cm −1 .

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of and claims priority toU.S. application Ser. No. 10/669,141, filed Sep. 23, 2003 (attorneydocket number BI4282), and hereby incorporates by reference the subjectmatter of that application.

BACKGROUND

1. Field of the Invention

The present invention is generally directed to materials and articleshaving a spinel crystal structure. In addition, the present inventionrelates generally to spinel materials particularly useful for opticalapplications.

2. Description of the Related Art

Various aluminous materials have been used and/or evaluated fordemanding optical applications. Such optical applications include, forexample, high powered lasing applications, in which the optical materialis utilized as a window or mirror, through which an optical laser beammay be passed or reflected. Aluminous materials that have been underconsideration include single crystal alumina, typically in the form ofsapphire. Other materials are microstructurally distinct from alumina,but containing a substantial portion of alumina groups, includingyittria alumina garnet (YAG), as well as spinel MgO·Al₂O₃). Whilesapphire and YAG demonstrate certain levels of robustness, the artcontinually demands materials having superior performance. In addition,sapphire does not have an optically isotropic structure, andaccordingly, careful attention must be paid during fabrication ofcomponents to properly align the microstructure with the intended axisof the light passing through the component.

Spinel-based materials have shown promise for use in demanding opticalapplications, such as military use of high powered lasers. However, suchmaterials are not without drawbacks, including materialfabrication/processing issues. In this regard, the industry has soughtto develop single crystalline spinel material, such as in the form ofboules, from melt-based process techniques including the so-calledCzochralski technique among others. Here, generally a stoichiometriccrystal (typically MgO·Al₂O₃, having an MgO:Al₂O₃ ratio of 1:1) is grownfrom a batch melt. While melt-based techniques have shown much promisefor the creation of single-crystal spinel materials, the process isrelatively difficult to control and suffers from undesirably low yieldrates, thereby increasing costs. In addition, extended cooling periodsand annealing periods are carried out to remove residual internalmechanical strain and stress present in the boules following bouleformation. Such cooling rates may be unusually low, and cooling periodssignificantly long, affecting throughput and increasing thermal budgetand cost. In a similar manner, the extended annealing times, which mayrange into the hundreds of hours, further increase processing costs.

Still further, even beyond the relatively high processing costs anddespite the precautions taken in an attempt to address residualmechanical strain and stress in the crystal, oftentimes the wafersformed from boules tend to suffer from undesirably high failure rates,with frequently lower than 20% yield rates.

In view of the foregoing, it is generally desirable to provide improvedspinel materials, well suited for optical applications, as well asimproved methods for forming same.

SUMMARY

According to one aspect, a single crystal spinel material is provided,the material having a non-stoichiometric composition and having atransparency window represented by absorptivity over a wavelength range,the wavelength range extending from about 400 nm to about 800 nm. Thetransparency window is defined as the largest single absorptivity peakheight along the wavelength range, the largest single peak height beingnot greater than 0.35 1/cm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the optical transmission (absorptivity) properties ofa previously developed cobalt-doped inverse spinel used in Q-switchingapplications.

FIG. 2 illustrates the optical transmission (absorptivity) properties ofan alumina rich spinel, according to an embodiment of the presentinvention.

FIG. 3 illustrates a portion of the curve shown in FIG. 2.

DETAILED DESCRIPTION

According to one aspect of the present invention, single crystal spinelmaterials, generally in the form of structural components, are provided.The single crystal spinel material generally has a non-stoichiometriccomposition and, according to one embodiment, has a transparency windowover a wavelength range. The wavelength range generally extends along atransmission range from about 400 nm to about 800 nm. The transparencywindow may be defined as the largest single absorptivity peak heightalong the wavelength range, generally not greater than about 0.35 cm⁻¹.According to certain embodiments, the wavelength range is furtherextended, meaning that the transparency window is maintained over awider frequency range. For example, the wavelength range may extend upto about 2000 nm such as 3000 nm, 3500 nm, or even 4000 nm. Theabove-noted largest single absorptivity peak height in certainembodiments is even further reduced, representing even superiortransmittance properties, such as a height not greater than about 0.33cm⁻¹, about 0.30 cm⁻¹, about 0.25 cm⁻¹, about 0.20 cm⁻¹, about 0.15cm⁻¹, or even about 0.10 cm⁻¹. Desirably, transmittance (or absorption)properties are fairly flat over an extended wavelength range, indicatinga lack of dependency of transmittance properties based upon wavelengthor frequency.

The actual optical transmittance measurements are dependent upon variousparameters. Generally, optical transmittance data are taken from sampleshaving a thickness within a range of about 5 to 10 mm, which samples aremachined for parallelism, flatness and surface finish. Samples had aparallelism less than 10 seconds or 0.003 degrees, flatness of {fraction(1/10)} wave maximum deviation over 90% of aperture as measured with a632.8 nm HeNe, and Mil spec, requiring scratch and dig specificationsaccording to Mil-O-13830A, having a 20/10 finish. However, the reportedabsorptivity data are intrinsically normalized for thickness of thesample, that is, are generally thickness independent.

To clarify the foregoing optical properties, attention is drawn to thedrawings herein. FIG. 1 illustrates the optical transmission data takenfrom a MgO·Al₂O₃ spinel having a b:a ratio of 3:1, doped with 0.01% ofCo²⁺. This particular material was formed according to an embodimentdescribed in U.S. patent application Ser. No. 09/863,013, published asU.S. Ser. No. 2003/0007520, commonly owned by the present assignee. Thisparticular material is used for Q-switching applications, generallydistinct from the optical applications according to embodiments of thepresent invention. As illustrated, the sample has a largest singleabsorptivity peak height of about 0.4 cm⁻¹ occurring at about 590 nm.

In contrast, FIGS. 2 and 3 illustrate the optical transmissionproperties according to an embodiment of the present invention, namelyan undoped aMgO·bAl₂O₃ spinel having a b:a ratio of about 3:1. Asillustrated, the sample has a fairly wide transmission window extendingfrom about 400 nm to about 3700 nm. The largest single absorptivity peakis less than about 0.1 cm⁻¹, occurring at about 800 nm, represents amuch smaller optical transmission loss or absorption than thecobalt-doped sample illustrated in FIG. 1. A similar absorptivity peakoccurs at about 3000 nm.

Turning to fabrication of spinel materials, typically, processing beginswith the formation of a batch melt in a crucible. The batch melt isgenerally provided to manifest a non-stoichiometric composition in theas-formed spinel material, generally in the form of a “boule,”describing a single crystal mass formed by melt processing, whichincludes ingots, cylinders and the like structures. According to oneembodiment, the boule has a general formula of aAD·bE₂D₃, wherein A isselected from the group consisting of Mg, Ca, Zn, Mn, Ba, Sr, Cd, Fe,and combinations thereof, E is selected from the group consisting Al,In, Cr, Sc, Lu, Fe, and combinations thereof, and D is selected from thegroup consisting O, S, Se, and combinations thereof, wherein a ratiob:a>1:1 such that the spinel is rich in E₂D₃. For clarification, astoichiometric composition is one in which the ratio of b:a=1:1, whilenon-stoichiometric compositions have a b:a ratio≠1:1.

According to certain embodiments, A is Mg, D is O and E is Al, such thatthe single crystal spinel has the formula aMgO·bAl₂O₃. While some of thedisclosure contained herein makes reference to the MgO—Al₂O₃ spinelbased-compositions, it is understood that the present disclosure moregenerally relates to a broader group of spinel compositions, having thegeneralized formula aAD·bE₂D₃, as described above.

While E₂D₃-rich spinels are generally represented by a ratio b:a greaterthan 1:1, certain embodiments have a b:a ratio not less than about1.2:1, such as not less than about 1.5:1. Other embodiments have evenhigher proportions of E₂D₃ relative to AD, such as not less than about2.0:1, or even not less than about 2.5:1. According to certainembodiments, the relative content of E₂D₃ is limited, so as to have ab:a ratio not greater than about 4:1. Specific embodiments may have ab:a ratio of about 3:1 (e.g., 2.9:1).

Following formation of a batch melt in a crucible, typically, the spinelsingle crystal boule is formed by one of various techniques such as theCzochralski pulling technique. While the Czochralski pulling techniquehas been utilized for formation of certain embodiments herein, it isunderstood that any one of a number of melt-based techniques, asdistinct from flame-fusion techniques, may be utilized. Such melt-basedtechniques also include the Bridgman method, the liquefied encapsulatedBridgman method, the horizontal gradient freeze method, and edge-definedgrowth method, the Stockberger method, or the Kryopolus method. Thesemelt-based techniques fundamentally differ from flame fusion techniquesin that melt-based techniques grow a boule from a melt. In contrast,flame fusion does not create a batch melt from which a boule is grown,but rather, provides a constant flow of raw material (such as in powderform), to a hot flame, and the molten product is then projected againsta receiving surface on which the molten product solidifies.

Generally, the single seed crystal is contacted with the melt, whilerotating the batch melt and the seed crystal relative to each other.Typically, the seed crystal is formed of stoichiometric spinel and hassufficiently high purity and crystallographic homogeneity to provide asuitable template for boule growth. The seed crystal may be rotatedrelative to a fixed crucible, the crucible may be rotated relative to afixed seed crystal, or both the crucible and the seed crystal may berotated. During rotation, the seed crystal and the actively formingboule are drawn out of the melt.

Typically, the boule consists essentially of a single spinel phase, withno secondary phases. According to another feature, the boule and thecomponents processed therefrom are free of impurities and dopants. Forexample, Co is restricted from inclusion in the foregoing embodiment,which otherwise is a dopant for Q-switch applications. In contrast toQ-switch applications, it is generally desired that a relatively purespinel is utilized substantially free of dopants that affect the basicand novel properties of the device substrates.

According to embodiments of the present invention, a single crystalspinel boule is formed having desirable properties. In addition todesired optical properties, the boules, and components formed therefromalso generally have reduced mechanical stress and/or strain, as comparedto stoichiometric articles having a b:a ratio of 1:1. In this regard,embodiments of the present invention provide desirably high yield ratesin connection with formation of single crystal components that formintegral parts of larger scale optical assemblies, and also provideimproved processing features, discussed in more detail hereinbelow.

With respect to improved processing features, the boule may be cooled atrelatively high cooling rates such as not less than about 50° C./hour.Even higher cooling rates may be utilized according to embodiments ofthe present invention, such as not less than about 100° C./hour, 200°C./hour and even at a rate of greater than about 300° C./hour. Theincreased cooling rates desirably improve throughput of the fabricationprocess for forming single crystal boules and further reduce the thermalbudget of the entire fabrication, and accordingly reduce costs. Boulesformed according to conventional processing generally are cooled atrelatively low cooling rates, in an attempt to prevent fracture duringthe cooling process. However, according to embodiments of the presentinvention, the cooling rates may be substantially higher yet stillprovide intact boules in the as-cooled form. Generally, conventionalcooling rates are on the order of 40° C./hour or less, requiring coolingperiods on the order of days.

Still further, according to another embodiment of the present invention,annealing of the boule, conventionally carried out subsequent tocooling, is restricted to a relatively short time period. Typically, thetime period is not greater than about 50 hours, such as not greater thanabout 30 hours, or even 20 hours. According to certain embodiments, theannealing is restricted to a time period not greater than about 10hours. Indeed, annealing may be substantially completely eliminated,thereby obviating post-forming heat treatment. In contrast, conventionalboule forming technology generally requires use of substantial annealperiods in an attempt to mitigate residual internal stress and strain,responsible for low wafer yield rates as well as boule fracture. Withoutwishing to be tied to any particular theory, it is believed that thereduction and internal stress and strain in the boule according toembodiments herein permits such flexible processing conditions,including decreased or complete elimination of annealing periods, aswell as increased cooling rates as noted above.

According to another feature, the reduction in internal mechanicalstress and strain are quantified by yield rate, the number of intactcomponents formed by machining the boule. Typically, machining iscarried out by any one of several slicing techniques, most notably wiresawing. As used herein, yield rate may be quantified by the formulac_(i)/(c_(i)+c_(f))×100%, wherein c_(i)=the number of intact componentsprocessed from the boule, and c_(f)=the number of fractured componentsfrom the boule due to internal mechanical stress or strain in the boule.Conventionally, this yield rate is very low, such as on the order 10%.The unacceptably low yield rate is a manifestation of excessive internalstresses and strain in the boule. In contrast, yield rates according toembodiments of the present invention are typically not less than about25%, 30% or even 40%. Other embodiments show increasingly high yieldrates, such as not less than about 50, 60 or even 70%. Indeed, certainembodiments have demonstrated near 100% yield. This reduce internalmechanical stress and/or strain as quantified above is not only presentwithin the as-formed (raw) boules, but also the processed boules, thecomponent machined from boules. In this regard, the foregoingdescription of processed boules generally denotes boules that have beensubjected to post-cooling machining steps, such as grinding, lapping,polishing and cleaning.

Turning to the particular physical manifestation of the spinelmaterials, embodiments may have various geometric configurations. Forexample, the material may be in the form of a polygonal planar windowsuch as a rectangle or square. Alternatively, the component may be inthe shape of a flat disc having a circular or oval outer periphery.Certain specialized applications call for more complex shapes, such asin the form of a cone or dome. Such components may be suitably utilizedat the leading end of a laser guided missile, for example. Still othermanifestations include light tubes, akin to fiber optic components. Aparticular application includes mirrors, having a highly polishedsurface oriented at a particular angle to reflect and/or transmit IRlight, in applications such as in lasing devices, particularly includingthe laser cavity.

Turning to durability testing, various materials were tested in acontrolled environment to determine user damage thresholds. Damagetesting was carried out by the so-called least fluence failuretechnique, utilizing a nominal pulse width (FWHM) of 20 ns, at anincidence angle of 0°. The number of sites utilized for testing wasvaried, generally within a range of 60 to 90. Shots per site were alsovaried, generally within a range of about 50 to 200. According to anembodiment of the present invention, typically, the material has a laserdamage threshold of not less than about 3.00 GW/cm² at a wavelength of1064 nm. The laser damage threshold may even be higher, such as not lessthan about 3.25, or even 3.50 GW/cm² at a wavelength of 1064 nm

A first set of data was generated at a wavelength of 1064 nm. The spotdiameter (1/e²) was 430 microns. 80 sites were tested at a rate of 200shots per site. Table 1 below summarizes the data of a 3:1 spinelaccording to an embodiment of the present invention, as contrastedagainst stoichimetric 1:1 spinel, as well as sapphire and YAG. TABLE 1Damage Threshold @ Damage Threshold @ MATERIAL 1064 nm (J/cm²) 1064 nm(GW/cm²) Sapphire 38.6 1.93 YAG 28.0 1.40 1:1 Spinel 51.7 2.58 3:1Spinel 80.0 4.00

As illustrated, the 3:1 spinel demonstrates superior damage resistanceto laser exposure, notably demonstrating an unexpected damage thresholdof 4.00 GW/cm².

Table 2 below summarizes the data for various samples at 1540 nm. Thetesting was carried out in a manner similar to the 1064 nm data. Here,the spot diameter was 115 microns. For the cobalt-doped sample, the spotdiameter was 170 microns and 50 shots per site were utilized rather than200 shots per site. TABLE 2 Damage Threshold @ Damage Threshold @MATERIAL 1540 nm (J/cm²) 1540 nm (GW/cm²) Sapphire 36.7 1.8 YAG 65.9 3.31:1 Spinel 118.0 5.9 3:1 Spinel 67.6 3.4 1:3 Spinel Co²⁺ 63.5 3.2

Further, damage threshold testing was carried out at a wavelength of 532nm. Again, testing was carried out in a manner similar to the 1064 nmtesting unless otherwise indicated. Here, a spot diameter of 300 micronswas utilized, a pulse width of 18 ns and the number of sites wasincreased to 100, while carrying out 200 shots per site. TABLE 3 DamageThreshold @ Damage Threshold @ MATERIAL 532 nm (J/cm²) 532 nm (GW/cm²)Sapphire 16.38 0.82 YAG 15.61 0.78 1:1 Spinel 44.97 2.25 3:1 Spinel 10.00.50

Still further, Table 4 below summarizes testing at 2100 nm. Testing wascarried out at a pulse width of 40 ns, a spot diameter of 140 μm. 50sites were tested, at a density of 200 shots/site. TABLE 4 DamageThreshold @ Damage Threshold @ MATERIAL 2100 nm (J/cm²) 2100 nm (GW/cm²)Sapphire 35.0 1.75 YAG 53.0 2.65 1:1 Spinel 60.0 3.0 3:1 Spinel 50.0 2.5

Still further, testing was carried out at 3000 nm. Testing was carriedout at a pulse width of 10 ns, a spot diameter of 110 μm. 40 sites weretested, at a density of 200 shots/site. TABLE 5 Damage Threshold @Damage Threshold @ MATERIAL 3200 nm (J/cm²) 3200 nm (GW/cm²) Sapphire35.0 1.75 YAG 48.8 2.44 1:1 Spinel >55.0 >2.75 3:1 Spinel >55.0 >2.75

EXAMPLE

Here, a specific process flow was utilized to create a single crystalspinel material according to an embodiment of the present invention.

Crucible Charge Preparation: 392.1 g of MgO were combined with 2876.5 gof Al₂O₃ (aluminum oxide). The raw materials were mixed together andheated for 12 hrs. At 1100 degrees centigrade in ceramic crucible. Aftercooling, the mixture was red into an iridium crucible 100 mm in diameterand 150 mm tall.

Crystal Growth: The iridium crucible with the oxide mixture was placedin standard Czochralski crystal growth station, and heated to themelting point of the oxide mixture by means of radio frequency heating.An inert ambient atmosphere consisting of nitrogen with a small additionof oxygen was used around the crucible.

After the mixture was liquid a small seed crystal of the 1:1 spinel with<111> orientation attached to the pulling rod was used to initiate thestart of the crystal growth process. A single crystal boule was grownutilizing the following process conditions, diameter 53 mm, length 150mm, seed pulling rate 2 mm/hr, seed rotation rate 4 rpm, cool-down time6 hrs, total time 123 hrs.

After cooling the crystal was visually inspected for bubbles, inclusionsor any other visible defects. After visual inspection the top and bottomends were removed, and crystal was subjected to an x-ray orientationcheck (Laue diffraction technique). After passing all inspection teststhe crystal was ready for fabrication.

The foregoing description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimit the scope to the precise form or embodiments disclosed, andmodifications and variations are possible in light of the aboveteachings, or may be acquired from practice of embodiments of theinvention.

1. A single crystal spinel material, the material having anon-stoichiometric composition and having a transparency windowrepresented by absorptivity over a wavelength range, the wavelengthrange extending from about 400 nm to about 800 nm, the transparencywindow being defined as the largest single absorptivity peak heightalong said wavelength range, the largest single peak height being notgreater than 0.35 cm⁻¹.
 2. The material of claim 1, wherein thewavelength range extends up to about 2000 nm.
 3. The material of claim1, wherein the wavelength range extends up to about 3000 nm.
 4. Thematerial of claim 1, wherein the wavelength range extends up to about3500 nm.
 5. The material of claim 1, wherein the wavelength rangeextends up to about 4000 nm.
 6. The material of claim 1, wherein theheight is not greater than about 0.30 cm⁻¹.
 7. The material of claim 1,wherein the height is not greater than about 0.25 cm⁻¹.
 8. The materialof claim 1, wherein the height is not greater than about 0.20 cm⁻¹. 9.The material of claim 1, wherein the material consists essentially of asingle spinel phase, with substantially no secondary phases.
 10. Thematerial of claim 1, wherein the material has the general formulaaAD·bE₂D₃, wherein A is selected from the group consisting of Mg, Ca,Zn, Mn, Ba, Sr, Cd, Fe, and combinations thereof, E is selected from thegroup consisting Al, In, Cr, Sc, Lu, Fe, and combinations thereof, and Dis selected from the group consisting O, S, Se, and combinationsthereof, wherein a ratio b:a>1:1 such that the material is rich in E₂D₃.11. The material of claim 10, wherein A is Mg, D is O, and E is Al, suchthat the material has the formula aMgO·bAl₂O₃, the material consistingessentially of aMgO·bAl₂O₃.
 12. The material of claim 11, wherein theratio b:a is not less than about 1.2:1.
 13. The material of claim 11,wherein the ratio b:a is not less than about 1.5:1.
 14. The material ofclaim 11, wherein the ratio b:a is not less than about 2.0:1.
 15. Thematerial of claim 11, wherein the ratio b:a is not less than about2.5:1.
 16. The material of claim 11, wherein the ratio b:a is about 3:1.17. The material of claim 11, wherein the ratio b:a is not greater thanabout 4:1.
 18. The material of claim 11, wherein the material has alower mechanical stress and strain compared to stoichiometric spinel.19. The material of claim 1, wherein the material has a laser damagethreshold of not less than about 3.00 GW/cm², at a wavelength of 1064nm.
 20. The material of claim 1, wherein the material has a laser damagethreshold of not less than about 3.25 GW/cm², at a wavelength of 1064nm.
 21. The material of claim 1, wherein the material has a laser damagethreshold of not less than about 3.50 GW/cm², at a wavelength of 1064nm.
 22. The material of claim 1, wherein the material is in the form ofan optical window.
 23. The material of claim 1, wherein the material isin the form of an optical mirror.
 24. The material of claim 1, whereinthe material is in the form of a light pipe.